§ 2, cl. I. "No principle is more fundamental to the judiciary's proper role in our system of government than [this] constitutional limitation[.]" Simon v. E. Ky Welfare Rts. Org., 426 U.S. 26, 37, 96 S.Ct. 1917, 48 L.Ed.2d 450 (1976). The prohibition against advisory opinions is "the oldest and most consistent thread